EDS is a disease of chickens characterized primarily by low egg production and abnormal egg production in chickens infected with EDSV (NPL1, and NPL 2). EDSV is classified as a member of the group III avian adenovirus, and has hemagglutination activity. The virus is characterized by proliferation in the chicken oviduct. EDS shows only minor clinical symptoms, such as mild diarrhea, if any. The disease often occurs in 30 to 40 weeks of age, where the egg production is at its peak, and causes production of eggs with abnormal shells, or low egg production over a period of 3 to 8 weeks in an infected flock. This leads to serious economic damage. Vaccines are effective for the prevention of EDS (NPL 3), and inactivated vaccines are incorporated in a wide range of vaccination programs for layers.
An EDS inactivated vaccine prepared by inactivating the viruses amplified in an embryonated domestic duck egg, and mixing the inactivated viruses with an adjuvant is currently the mainstream. While the EDSV amplification in an embryonated domestic duck egg is desirable (NPL4, and NPL 5), the emergence of avian influenza and other infections have raised concerns that the future supply of embryonated domestic duck eggs might be difficult. There accordingly is a need for a means to supply vaccine antigens that does not depend on domestic duck eggs.
EDSV has double-stranded genomic DNA of about 33 kb (NPL6). EDSV has a single serotype, and there is hardly any difference in antigenicity between strains (NPL 7). The virus particles are constructed from various structural proteins, including the core protein, the constituent hexone of the capsid, the trimeric fibers projecting out of the capsid as fibrous spikes, and a base portion, such as a penton base, providing a base for the fibers (NPL 1, and NPL 6). The neutralizing epitope is present in the hexone (NPL 8), the fiber, and the penton base (NPL 9). The fiber, in particular, has been examined in many studies (NPL 11) because of its involvement in entry into host cells (NPL 10). The fiber protein is divided into three regions by function (NPL 12). The N-terminal side involved in the linkage to the penton base (NPL 13) is called the tail, and the region in the middle is called the shaft. It is reported that the Knob at the C terminal has hemagglutination activity, and is involved in the binding to the cell receptor (NPL 11). In human adenovirus 5, the neutralizing epitope is present in the knob, one of the three regions of the fiber protein. There is a report that conformation is important to exhibit function, and that binding activity for the receptor was observed upon expression with a part of the adjacent shaft region (NPL 14).
The shaft region constituting the shaft of the fiber protein has a repeat structure with a series of multiple β spiral structures (1 repeat) including hydrophobic regions separated by proline or glycine (NPL 12, and NPL 15), and three molecules are assembled together to form a triple β spiral structure (NPL 15, and NPL 16). The knob following the shaft also has a trimeric structure (NPL 17). Typically, a shaft repeat consists of 15 amino acids (NPL 12). In EDSV, however, most repeats consist of larger numbers of amino acids (NPL 6). In human adenovirus, it is reported that reducing the shaft length leads to weaker binding to the receptor, and reduced infectivity for culture cells (NPL 18).
Concerning EDSV, there are reports that neutralizing antibodies were induced in a chicken to which the Knob, and 34 amino acids at the C-terminal portion of the shaft region adjacent to the knob were administered as an oil vaccine after expression in Escherichia coli (NPL 19 and NPL 20, and PTL 1).